The subject matter disclosed herein relates to a turbine shell with pin support.
In gas turbines, inner turbine shells support nozzles and shrouds radially and axially with respect to a turbine rotor. The concentric support structure between the nozzles, the shrouds, and the rotor extends from the rotor bearing, to the exhaust frame, to the outer turbine shell, to the inner turbine shell and to the nozzles and the shrouds themselves. The rotor bearing is supported by the exhaust frame, which, in turn, is connected to grounded support with support legs and a gib providing engine support and stability. In addition, configurations that include a combination of inner and outer turbine shells provide additional clearance due to relative thermal response between the stator and rotor and structural isolation between the inner and the outer turbine shell.
Generally, active clearance controls are employed to radially displace inner and outer turbine shells from one another during turbine operations. This has the effect of controlling tip clearance between buckets and shrouds, which can be useful since decreasing tip clearance improves turbine performance by reducing tip leakage as long as bucket tips are prevented from contacting and thereby damaging shrouds.
Even with active clearance controls, however, in some configurations relative movement occurs between the inner and outer turbine shells due to differential thermal growth of their respective components. To reduce eccentricity caused by the relative movement, the inner turbine shell may be supported with radial pins attached to the outer turbine shell or by the use of complementary radial surfaces between the outer and inner turbine shells. In such configurations, an assembly clearance gap exists between the radial supports to prevent binding during engine operation.
In any case, when relative movement between the inner and outer turbine shells occurs, leakage paths are formed and frictional forces are generated. These frictional forces can lead to damage, such as contact surface wear on mating surfaces, which occurs during thermal expansion and contraction of either the inner or the outer turbine shell. That is, during expansion and contraction, the components experience static and dynamic frictional contact. At the same time, the friction coefficient of the components vary significantly and unpredictably. As a result, the frictional forces that impede radial displacement of the inner turbine shell relative to the outer turbine shell also vary. This variation causes the position of the inner turbine shell to shift toward and stick to the high friction locations. This friction effect combined with the assembly clearances leads to shell eccentricity that is often indeterminate within allowable clearances.
Additionally, stator tube casings are generally split at the horizontal mid-plane and incorporate a bolted flange at this horizontal joint. Thermal gradients and transient boundary conditions create an inherent out-of-roundness of the entire casing. When the inner portions are hotter than the outer portions, as is found during engine startup, such casings assume a football shape. Conversely, during engine shut down, the outer portions are warmer than the inner portions, causing the casing to assume a peanut shape. Such out-of-roundness is transmitted through the stator tube to the shrouds causing gaps between the shrouds and bucket tips, decreasing engine performance.
Shell out-of-roundness is also a problem in steam turbines. In these cases, occurrences of shell out-of-roundness may be due to a horizontal joint in the turbine shell, which acts as a heat sink and creates perimetrical variation in shell temperature. The temperature variation causes the shell to distort or ovalize. That is, the shell exhibits a greater dimension in the vertical direction than in the horizontal. The rotor, in contrast, remains circular. The ovalized shape of the shell results in increased clearances, and hence more leakage than if the stator remained circular.